1. Field of the Invention
The present invention relates to an optical head used in optical information processing, optical communication or the like and an optical recording and reproducing apparatus using the optical head.
2. Description of the Related Art
Recently, a digital versatile disc (DVD) has attracted attention as a high-capacity optical recording medium because it can record digital information in a recording density which is about 6 times as high as a compact disc (CD). However, a further high-density optical recording medium is demanded as capacity of information becomes large. Here, in order to realize a density higher than the DVD (wavelength is 660 nm and numerical aperture (NA) is 0.6), it is necessary to use a light source emitting a light having shorter wavelength and to further increase the NA of the objective lens. For example, when blue laser having a wavelength of 405 nm and an objective lens having NA of 0.85 are used, a recording density which is 5 times as high as the DVD can be attained.
However, since the high-density optical recording medium apparatus using the blue laser has very strict reproducing and/or recording margin, in other words, a permissible level for a fluctuation of characteristic in reproducing or recording is limited very strictly, aberration generated when the optical recording medium is tilted becomes a problem. It is to be noted that the wording “reproducing and/or recording” means “at least one of reproducing and recording”, in the specification, to simplify the description.
In relating to this problem, Japanese Patent Laid-open Publication No. 11-312327 discloses an optical head which can perform reproducing and recording operations by tilting an objective lens corresponding to the tilt of the optical recording medium to reduce the aberration.
One example of the above conventional optical head is described with reference to the drawing.
FIG. 7 is a schematic view showing a constitution of the conventional optical head. In FIG. 7, reference numeral 61 designates a light source, reference numeral 62 designates a collimator lens, reference numeral 63 designates a beam splitter, reference numeral 64 designates a raising mirror, reference numeral 65 designates an objective lens, reference numeral 66 designates an optical recording medium, reference numeral 67 designates a detection optical system, reference numeral 68 designates a lens holding member, reference numeral 69 designates a tilt sensor, reference numeral designates a focus error signal detection circuit, reference numeral 71 designates a tracking error signal detection circuit, reference numeral 72 designates a reproduction signal detection circuit, reference numeral 73 designates a tilt detection circuit, reference numeral 74 designates a controller, and reference numeral 75 designates a tilt control circuit.
The light source 61 is a semiconductor laser and it outputs recording and reproducing coherent light to a recording layer of the optical recording medium 66. The collimator lens 62 converts the light emitted from the light source 61 to parallel light, the beam splitter 63 is an optical element for isolating light, the raising mirror 64 is an optical element for reflecting the input light so as to orient it toward the optical recording medium, the objective lens 65 converges the light to the recording layer of the optical recording medium 66. The lens holding member 68 is to hold the objective lens 66 and capable of tilting the objective lens 66. And, the tilt sensor 69 is to detect the tilt of the optical recording medium 66.
Operation of the optical head constituted as described above is explained. The linear polarized light emitted from the light source 61 is converted to the parallel light by the collimator lens 62. The parallel light passes through the-beam splitter 63 and it is reflected by the raising mirror 64 and converged onto the optical recording medium 66 by the objective lens 65. Then, the reflected light from the optical recording medium 66 passes through the objective lens 65 and it is reflected by the mirror 64, reflected by the beam splitter 63 and led to the detection optical system 67. Here, a focus error signal, a tracking error signal and reproduction signal are detected. The focus error signal and the tracking error signal are detected by the well-known technique such as astigmatic method, a push-pull method or the like. If necessary, an offset is applied to the focus error signal and the tracking error signal by the controller 74. Focus controlling device (not shown) moves and controls the position of the objective lens 65 in the light-axis direction based on the focus error signal so that the light may be always converged onto the optical recording medium 66 in a focused state. In addition, tracking controlling device (not shown) moves and controls the position of the objective lens 65 based on the tracking error signal so that the light may be converged onto a desired track on the optical recording medium 66.
The tilt control of the objective lens 65 is performed based on a tilt signal detected by the tilt sensor 69 which detects the tilt of the optical recording medium 66 and the objective lens 65. The tilt sensor 69 is provided beside the objective lens 65. The tilt signal detected by the tilt sensor 69 is input to a tilt control circuit 75. And a signal for tilting the objective lens 65 is output from the tilt control circuit 75, the lens holding member 68 is controlled so as to eliminate the tilt of the objective lens 65 and the optical recording medium 66.
In such constitution, even when the optical recording medium 66 is tilted, the aberration can be reduced by detecting its tilt amount and tilting the objective lens 65, so that reproducing and/or recording operations can be stably performed.
However, according to the conventional optical head having the above constitution, when the objective lens itself has coma aberration, a tilt correcting function deteriorates in some cases. This is described in detail hereinafter. For example, it is assumed that recording and/or reproducing operations are performed on a optical recording medium having a substrate thickness of 0.1 mm with an optical head in which a wavelength of a light source is 405 nm and NA of an objective lens is 0.85. Here, since the optical recording medium is warped in the radial direction as is well known, it is assumed that only radial tilt correction is made. Since the objective lens has very large NA such as 0.85, coma aberration is considerably generated by tolerance (surfaces tilt, decenter of surface or the like) at the time of assembly. Thus, when the objective lens is mounted on the optical head and the optical head is assembled without considering the coma aberration owned by the objective lens itself, the direction of the coma aberration of the objective lens coincides with the radial direction in some cases, for example. When it is assumed that the coma aberration of the objective lens is 30 mλ and the above optical head is assembled, the objective lens is tilted by 0.3 degree in the radial direction (that is, so as to be tilted along the radial direction) in order to eliminate the coma aberration of 30 mλ and mounted on the optical head. When the warp of the optical recording medium is corrected by the optical head assembled in such state, since the objective lens has been tilted in an initial state, it is effective for the warp in one direction but it is necessary to largely tilt the objective lens for the warp in the other direction. Therefore, aberration other than the coma aberration, such as a spherical aberration, is generated differently even when the warp amount is the same if the direction of the warp is different, whereby the tilt correcting function deteriorates.